In wireless communication networks, users of communication devices typically move around with their communication device(s), hereinafter referred to as wireless device(s). Some non-limiting examples of a wireless device are a User equipment, UE, a Personal Digital Assistant, PDA, a laptop, or any other apparatus such as e.g. a vehicle comprising communication means for communicating with a node or entity of the wireless communication network.
As the locations of the wireless devices change, so do the radio conditions as the path between the wireless device and a serving network node changes. Further, the number of different wireless devices currently being served by, or connected to, the serving network node changes irregularly.
In order to cope with the constantly changing radio conditions, the serving network node has several options and functions in order to try to keep the quality of each radio channel between itself and a respective wireless device to be as good as possible, or at least good enough to fulfil some quality of service requirements.
One example of a function to improve, or keep, the channel quality to an acceptable level is link adaptation, LA. In order to enable link adaptation, a wireless device measures on downlink reference symbols transmitted by network nodes and predicts best rank and Modulation and Coding Scheme, MCS, which is an example of a transmission parameter. The wireless device may further suggest a precoder. Proposed rank is reported as Rank Indicator, RI, and proposed MCS as Channel Quality Indicator, CQI.
The CQI may be noisy and can be more or less outdated when used in the serving network node for MCS selection. An outer-loop is used to adapt to the impact of this and compensate for channel changes and movement of the wireless device. The outer-loop is typically a Block Error Rate, BLER,-based jump algorithm that targets a certain Hybrid Automatic Repeat Request, HARQ, BLER, e.g. 10%, adjusting with a margin in dB for the CQI inaccuracy. The adjustment is thus continuously updated as long as the wireless device is scheduled in the downlink, and becomes higher if one or more ACKs are received, and lower if the wireless device reports one or more NACKs. The outer-loop also adapts to other measurement noise such as wireless device vendor implementation differences and measurement errors.
Networks using Transmission Mode 4, TM4, and colliding Cell Specific Reference Signals, CRSs, are not unlikely to continue to be important in the future, due to the lower complexity compared to e.g. TM10. Legacy wireless device using TM4 and colliding CRSs will also continue to exist for a long time due to the slow turnover time to change the existing wireless device population base. However, for TM4, most wireless device vendor implementations estimate the interference based on CRSs, which can result in a large interference estimation bias at low load. Interference estimation based on CRSs can be seen as always estimating the interference as coming from fully loaded neighbouring cells since CRSs are always transmitted, even if there is no data to transmit, see FIG. 1. The large interference bias may occur in any system where the CSI information is based on colliding CRSs, e.g. TM4 and TM9. See also FIG. 1.
For non-colliding CRSs, the interference estimation bias is generally much smaller since an interference measurement on a CRS resource captures Physical Downlink Shared Channel, PDSCH, transmissions from the surrounding network nodes with different CRS shift. However, colliding CRSs are typically preferred since non-colliding CRSs cause interference to the PDSCH transmissions.
Currently, the interference bias is often left for the outer loop LA to handle, but a more preferable solution to the problem with interference estimation bias is coordinated link adaptation. In a coordinated cluster, the serving network node may have knowledge about the PDSCH scheduling and may determine the interference based on if interfering network nodes are currently transmitting and the estimated received power of the interference.
The outer loop usually requires operation in small increments for stability reasons. It can take a long time to converge towards a proper adjustment if the raw reported CQI is not an accurate representation of the channel quality for demodulation, which is the case when the CSI information is based on colliding CRSs due to the interference estimation bias at low load. The difference between SINR based on CRS and SINR for demodulation may also change due to mobility or when the wireless device is in DTX for a longer period, which means the CQI adjustment stored in the serving network node will represent an outdated bias compensation that may not be suitable for the radio conditions.
Coordinated link adaptation requires a very good backhaul between the coordinated network nodes and a considerable study and implementation effort for it to work.